The person who said sugar does not cause diabetes is correct. People with diabetes cannot handle sugar, and sugar will cause all of their symptoms, but not cause the disease. Genetics and being over-weight are the main sources of the disease.
Type 1 Diabetes means that your pancreas is incapable of properly regulating sugar levels in the blood, due to its inability to properly create insulin. Before insulin replacement therapy was invented, these people died in droves

Type 2 Diabetes means the person is not only (almost always) overweight, but also have a genetic predisposition to insulin resistance. Over time, this resistance will grow, and this resistance can be accelerated by a high-sugar diet (both directly, and indirectly, the latter referring to the fact that sugar can help you become overweight). BUT, and this is the crux of the argument, BEING FAT CAUSES EXTRA HORMONE PRODUCTION WHICH INCREASES YOUR RESITANCE WITH TIME, and the body can convert non-sugars into sugars, meaning a person with the genetic predisposition AND extra weight on their body can become diabetic, sugar or no.
Anyone with a completely healthy pancreas and no genetic predisposition can pretty much eat as much sugar as they want, with no fear of diabetes, even if they get fat. If you are a man and your fat goes all to your belly, this is a warning sign of a potential predisposition towards diabetes (men who have their fat spread out more evenly are far less likely to get diabetes than their counter-parts who have the fat concentrate on their bellies). So, sugar can exacerbate and can even accelerate diabetes, but CANNOT cause it.
EDIT: In case acceleration and exacerbation sound a lot like causation, compare and contrast with tobacco smoking. Someone with genetic predisposition to Lung Cancer will probably get lung cancer whether they smoke or not. Smoking will accelerate their condition. But even among people with no disposition towards lung cancer whatsoever, and even among people with a disposition toward resilient, healthy lungs, smoking can STILL cause lung cancer, and as such smoking is a true CAUSE of the disease. Sugar will not generate diabetes in completely healthy people (where "healthy" here refers to no disposition towards diabetes), so it is not a CAUSE of disease, unlike smoking.